Backside illuminated image sensor pixel structure and image sensor thereof

Abstract

According to the backside illuminated image sensor pixel structure and the image sensor thereof provided by the invention, the first photodiode is arranged below the second photodiode, and the sensitivity of the first photodiode is smaller than that of the second photodiode, so that most of optical signals are absorbed after passing through the second photodiode; in the embodiment of the invention, the exposure time is controlled to ensure that the first photodiode is not overexposed and the exposure time covers an image signal with a relatively long stroboscopic period, and the exposure time can be shortened to ensure that the second photodiode is not overexposed; and therefore, when the exposure time is controlled to ensure that the first photodiode is not overexposed, the exposure time can cover the image signal with a relatively long stroboscopic period, and the exposure time is shortened to ensure that the second photodiode is not overexposed. Besides, in the pixel array, due to the fact that the position of the first photodiode is deep and the sensitivity is low, the first photodiode hardly absorbs crosstalk light rays from the light rays entering the first pixel units from the pixel units adjacent to the first pixel units at a large angle, and therefore the anti-crosstalk capacity of the first photodiode is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor manufacturing, and in particular to a back-illuminated image sensor pixel structure and an image sensor thereof. Background Art

[0002] With the development of chip integration, CMOS sensors are evolving towards smaller size and lower power consumption, and are increasingly being used in monitoring, automotive, and artificial intelligence fields. At the same time, the requirements for certain performance of CMOS image sensors in different application scenarios are also becoming increasingly higher.

[0003] In the field of vehicle monitoring, some image sensors need to monitor scenes outside the vehicle, which involves traffic lights. Usually, traffic lights are a kind of stroboscopic LED, and the chip needs to have a long enough exposure time to cover the LED's stroboscopic cycle to capture the image information of the LED "bright". However, the scene outside the vehicle will contain some bright signals, such as the bright light at the tunnel exit, the sun in the sky, and the headlights of nearby cars. In order to prevent some bright signals from being lost in the monitoring screen, the image sensor needs a relatively short exposure time, which is contrary to the requirement of a long enough exposure time to capture the flashing LED information. Therefore, the chip needs another LFM (Light Flicker Mitigation), that is, LED flicker suppression function, so that the bright scene is not overexposed while capturing the signal of the LED with a relatively long stroboscopic cycle.

[0004] In the prior art, LFM is achieved by adding anti-reflection materials to a portion of normal pixels, reducing the transmittance while controlling the exposure time of these pixels to cover the LED strobe cycle. However, the anti-crosstalk capability of such pixels is relatively weak. In addition, an additional process of filling anti-reflection materials is required in the manufacturing process, which increases the cost and process complexity. Summary of the invention

[0005] In view of the shortcomings of the prior art mentioned above, the object of the present invention is to provide a back-illuminated image sensor pixel structure and an image sensor thereof, which are used to solve the problems in the prior art that the back-illuminated image sensor realizes LFM by adding anti-reflection materials on a part of normal pixels, reducing the transmittance and controlling the exposure time of these pixels to cover the LED strobe cycle, such as complex process, high cost and weak anti-crosstalk ability.

[0006] To achieve the above-mentioned object and other related objects, the present invention provides a back-illuminated image sensor pixel structure, wherein the pixel structure includes a pixel array in which pixel units are arranged in an array, wherein all the pixel units in the pixel array include a plurality of first pixel units, and the first pixel units are evenly distributed in the pixel array, and the first pixel units include:

[0007] A semiconductor structure layer, wherein the semiconductor structure layer includes a first photodiode and a second photodiode in sequence from bottom to top, wherein the sensitivity of the first photodiode is smaller than the sensitivity of the second photodiode;

[0008] A circuit connection layer is formed with a first transfer gate and a second transfer gate, which are respectively coupled to the first photodiode and the second photodiode to transfer data signals accumulated in the first photodiode and the second photodiode in response to transfer signals applied to the first transfer gate and the second transfer gate.

[0009] Optionally, all the pixel units in the pixel array are set as the first pixel units.

[0010] Further, a projection of the second photodiode toward the first photodiode completely covers the first photodiode.

[0011] Furthermore, the volume of the first photodiode is smaller than the volume of the second photodiode.

[0012] Furthermore, the surface area of ​​the first photodiode is not greater than 50% of the surface area of ​​the second photodiode, and the depth of the first photodiode is within a range greater than 70% of the depth of the semiconductor structure layer.

[0013] Optionally, a surface of the second photodiode close to the first photodiode is stepped, and the first photodiode is located below an inner stepped surface of the stepped second photodiode.

[0014] Optionally, the first pixel unit further includes a charge overflow capacitor, and is coupled in parallel with the first photodiode; the charge overflow capacitor is a MOS capacitor arranged in the semiconductor structure layer or a MIM capacitor arranged in the circuit connection layer.

[0015] Further, the first photodiode corresponds to a first readout circuit arranged in the circuit connection layer, and the second photodiode corresponds to a second readout circuit arranged in the circuit connection layer, so as to respectively output the data signals corresponding to the first photodiode and the second photodiode; or, the first photodiode and the second photodiode share a readout circuit arranged in the circuit connection layer, so as to output the data signals corresponding to the first photodiode and the second photodiode based on the readout circuit.

[0016] Further, when the first photodiode corresponds to the first readout circuit and the second photodiode corresponds to the second readout circuit:

[0017] The first readout circuit and the second readout circuit each include a floating diffusion, a reset transistor, a source follower transistor and a gate transistor, respectively; wherein,

[0018] The reset transistor resets the floating diffusion;

[0019] The floating diffusion stores the data signal transferred from the first photodiode and the second photodiode; the gate transistor reads out the corresponding data signal through the source follower transistor;

[0020] The second readout circuit further includes a low conversion gain capacitor and a gain control transistor, wherein the gain control transistor is coupled between the floating diffusion and the reset transistor, one end of the low conversion gain capacitor is coupled to a node between the reset transistor and the gain control transistor, and the other end is grounded, so as to store the data signal overflowed from the second photodiode in the low conversion gain capacitor;

[0021] Or, when the first photodiode and the second photodiode share the readout circuit:

[0022] The readout circuit includes a floating diffusion, a reset transistor, a low conversion gain capacitor, a gain control transistor, a source follower transistor and a gate transistor; wherein,

[0023] The reset transistor resets the floating diffusion;

[0024] The floating diffusion stores the data signal transferred from the first photodiode and the second photodiode; the gate transistor reads out the corresponding data signal through the source follower transistor;

[0025] The gain control transistor is coupled between the floating diffusion and the reset transistor, one end of the low conversion gain capacitor is coupled to the node between the reset transistor and the gain control transistor, and the other end is grounded to store the data signal overflowed from the second photodiode in the low conversion gain capacitor.

[0026] The present invention further provides a back-illuminated image sensor, wherein the back-illuminated image sensor comprises a back-illuminated image sensor pixel structure as described in any one of the above.

[0027] As described above, the back-illuminated image sensor pixel structure and image sensor of the present invention, by arranging the first photodiode below the second photodiode in the first type of pixel unit and making the sensitivity of the first photodiode smaller than the sensitivity of the second photodiode, allows a normal light signal to enter the semiconductor structure layer through the optical structure layer, and most of the light signal is absorbed after passing through the second photodiode and converted into photoelectrons of the second photodiode, while a very small part of the light signal is not absorbed and finally reaches the first photodiode. Therefore, for image signals with relatively long stroboscopic periods, the exposure time of the first transfer gate can be controlled to ensure that the first photodiode is not overexposed while also ensuring that the first photodiode is not overexposed. The exposure time can ensure that the exposure time covers the image signal with a relatively long stroboscopic period, thereby realizing LFM. For the high-brightness image signal, the exposure time of the second transfer gate can be shortened to ensure that the second photodiode is not overexposed, so as to achieve the synchronous collection of the high-brightness signal and the signal with a long stroboscopic period. In addition, in the pixel array, the light from the pixel unit adjacent to the first pixel unit is incident on the first pixel unit at a large angle. Since the first photodiode is located deeper, its considerable optical path is in the photodiode above the first photodiode, and the sensitivity of the first photodiode is relatively low, resulting in the first photodiode hardly absorbing the crosstalk light, thereby effectively improving the anti-crosstalk capability of the first photodiode. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the cross-sectional structure of the pixel structure of the back-illuminated image sensor of the present invention, wherein the optical path of the image light in the first pixel unit is schematically shown.

[0029] Figure 2 A graph showing the relationship between the light flux of different wavelengths and depth in silicon medium.

[0030] Figure 3 Display as Figure 1 A light path diagram showing the light path of the first pixel unit in the back-illuminated image sensor pixel structure to mitigate the crosstalk of the pixel unit on the right side.

[0031] Figure 4 Display as Figure 1 FIG. 1 is a light path diagram of the first pixel unit in the back-illuminated image sensor pixel structure to mitigate the crosstalk of the pixel unit on the left side. FIG.

[0032] Figure 5 Shown is a circuit structure diagram of a first example of a back-illuminated image sensor pixel structure of the present invention.

[0033] Figure 6 Shown is a circuit structure diagram of a second example of a back-illuminated image sensor pixel structure of the present invention.

[0034] Figure 7 Shown is a circuit structure diagram of a third example of a back-illuminated image sensor pixel structure of the present invention.

[0035] Figure 8 Shown is a circuit structure diagram of a fourth example of a back-illuminated image sensor pixel structure of the present invention.

[0036] Figures 9 to 13 It is a schematic diagram of a cross-sectional structure after each step in a method for preparing a pixel structure of a back-illuminated image sensor according to an example of the present invention is completed.

[0037] Component number description

[0038] 10 Circuit connection layer

[0039] 100 First transfer gate

[0040] 101 Second transfer gate

[0041] 11 Semiconductor structure layer

[0042] 110 First photodiode

[0043] 111 Second photodiode

[0044] 112 Deep Trench Isolation

[0045] 113 Shallow Trench Isolation

[0046] 12 Optical structure layer

[0047] 120 Color Filters

[0048] 121 Optical Isolation Structure

[0049] 122 Microlens

[0050] 123 light-transmitting medium layer

[0051] 13 First pixel unit

[0052] 14 Charge overflow capacitor

[0053] 15. First readout circuit

[0054] 150, 160, 170 Floating diffusion

[0055] 151, 161, 171 Reset transistor

[0056] 152, 162, 172 Source follower transistor

[0057] 153, 163, 173 Gate transistor

[0058] 164, 174 Low conversion gain capacitors

[0059] 165, 175 Gain Control Transistor

[0060] 16. Second readout circuit

[0061] 17 Readout Circuit DETAILED DESCRIPTION

[0062] The following describes the embodiments of the present invention through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0063] See also Figures 1 to 13 It should be noted that the illustrations provided in this embodiment are only used to illustrate the basic concept of the present invention in a schematic manner, and the illustrations only show components related to the present invention rather than being drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component may be changed arbitrarily, and the component layout may also be more complicated.

[0064] Embodiment 1

[0065] As described in the background technology, in order to achieve LFM in image sensors, anti-reflection materials are usually added to some normal pixels to reduce the transmittance and control the exposure time of these pixels to cover the LED strobe cycle, but the anti-crosstalk ability of such pixels is relatively weak. The inventors studied and analyzed that the reason for the relatively weak anti-crosstalk ability is that after setting the anti-reflection material, the signal collected by the pixel from the front will be greatly reduced, but the crosstalk signal obliquely incident from the adjacent pixels will not be weakened, so that the relative proportion of the crosstalk signal in the signal collected by such pixels is greatly increased, causing the anti-crosstalk ability of such pixels to be reduced, affecting the imaging effect of such pixels and the subsequent image synthesis with normal pixels.

[0066] Based on the above analysis, this embodiment provides a back-illuminated image sensor pixel structure, such as Figure 1 As shown, the pixel structure includes a pixel array in which pixel units are arranged in an array, all the pixel units in the pixel array include a plurality of first pixel units 13, and the first pixel units 13 are evenly distributed in the pixel array, and the first pixel units 13 include:

[0067] A semiconductor structure layer 11, wherein the semiconductor structure layer 11 includes a first photodiode 110 and a second photodiode 111 in sequence from bottom to top, wherein the sensitivity of the first photodiode 110 is smaller than the sensitivity of the second photodiode 111;

[0068] A circuit connection layer 10 is formed with a first transfer gate 100 and a second transfer gate 101, which are respectively coupled to the first photodiode 110 and the second photodiode 111, that is, the first transfer gate 100 is coupled to the first photodiode 110, and the second transfer gate 101 is coupled to the second photodiode 111, so as to transfer out the data signals accumulated in the first photodiode 110 and the second photodiode 111 respectively in response to the transfer signals applied to the first transfer gate 100 and the second transfer gate 101.

[0069] It should be noted here that the up and down directions in this embodiment are defined by the direction of light from top to bottom, that is, the direction from top to bottom refers to the direction from the optical structure layer 12 to the semiconductor structure layer 11 and then to the circuit connection layer 10. Therefore, the direction from bottom to top in the semiconductor structure layer 11 is the opposite direction of light, that is, the direction from the semiconductor structure layer 11 toward the optical structure layer 12.

[0070] Depend on Figure 2 It can be seen that light of different wavelengths will be gradually absorbed and the light intensity will decrease as the light propagation depth increases in the same medium. Figure 2The medium shown in is a silicon medium. In the visible light range, when the propagation depth of light reaches 2μm, the light intensity attenuates by more than 50%. Therefore, in this embodiment, the first photodiode 110 is arranged below the second photodiode 111 in the first type of pixel unit and the sensitivity of the first photodiode 110 is made smaller than the sensitivity of the second photodiode 111, so that the normal light signal enters the semiconductor structure layer 11 through the optical structure layer 12. Most of the light signals are absorbed after passing through the second photodiode 111 and converted into photoelectrons of the second photodiode 111, while a very small part of the light signals are not absorbed and finally reach the first photodiode 110. Since the first photodiode 110 is located at a relatively deep position in the semiconductor structure layer 11 and has a relatively small sensitivity, the photosensitivity of the first photodiode 110 is relatively weak. Therefore, for image signals with relatively long stroboscopic periods, such as the stroboscopic period of LEDs, since the photosensitivity of the first photodiode 110 is relatively weak, the exposure time of the first transfer gate 100 can be controlled to ensure that the first photodiode 110 is not overexposed while ensuring that the exposure time covers the image signals with relatively long stroboscopic periods, thereby realizing LFM. For high-brightness image signals, the exposure time of the second transfer gate 101 can be shortened to ensure that the second photodiode 110 is not overexposed, thereby achieving the synchronous collection of high-brightness signals and signals with long stroboscopic periods. In addition, if Figure 3 and Figure 4 As shown, in the pixel array, the pixel unit adjacent to the first pixel unit 13 can be a normal pixel unit or the first pixel unit 13. The light incident on the first pixel unit 13 at a large angle from the pixel unit adjacent to the first pixel unit 13, due to the deeper position of the first photodiode 110, has a relatively long optical path in the photodiode above the first photodiode 110. For example, when the pixel unit adjacent to the first pixel unit 13 is a normal pixel unit, the light has a relatively long optical path in the photodiode in the normal pixel unit. When the pixel unit adjacent to the first pixel unit 13 is also the first pixel unit 13, the light has a relatively long optical path in the second photodiode 111 in the first pixel unit 13. In addition, the sensitivity of the first photodiode 110 is relatively low, resulting in the first photodiode 110 hardly absorbing the crosstalk light, thereby effectively improving the anti-crosstalk capability of the first photodiode 110.

[0071] The number of the first pixel units 13 in the pixel array is set according to practical needs, but needs to be evenly distributed in the pixel array to improve the imaging effect and subsequent image synthesis with normal pixel units. From the perspective of process preparation and circuit control, preferably, all the pixel units in the pixel array can be set as the first pixel units 13 to reduce the complexity of process and circuit control.

[0072] In one example, if Figure 1 As shown, the projection of the second photodiode 111 toward the first photodiode 110 completely covers the first photodiode 110, that is, the surface of the second photodiode 111 completely covers the surface of the first photodiode 110, so that the light enters the first photodiode 110 after entering the second photodiode 111, effectively improving the light utilization rate, thereby also improving the filling factor of the pixel unit. Further, the volume of the first photodiode 110 can be made smaller than the volume of the second photodiode 111, further reducing the sensitivity of the first photodiode 110. On this basis, the surface area of ​​the first photodiode 110 is set to be no more than 50% of the surface area of ​​the second photodiode 111, and the depth of the first photodiode 110 is preferably located in a range of more than 70% of the depth of the semiconductor structure layer 11.

[0073] like Figure 3 and Figure 4 As shown, when all the pixel units of the pixel array are set to the first pixel unit 13, the projection of the second photodiode 111 toward the first photodiode 110 completely covers the first photodiode 110, and the volume of the first photodiode 110 is smaller than the volume of the second photodiode 111, the side of the second photodiode 111 close to the first photodiode 110 can be set to be stepped, for example Figure 3 and Figure 4 The semiconductor structure layer 11 is configured as a three-step staircase, wherein the direction from shallow to deep along the thickness direction of the semiconductor structure layer 11 is the direction from inside to outside of the staircase surface, and the first photodiode 110 is located below the inner side staircase surface of the second photodiode 111. Figure 3 As shown, when the light intensity from the first pixel unit 13 on the right side of the left first pixel unit 13 is incident at a large angle to the first pixel unit 13 on the left, it will be attenuated by the second photodiode 111 in the first pixel unit 13 on the right and then enter the first photodiode 110 after being attenuated by the second photodiode 111 in the first pixel unit 13 on the left. After two attenuations, the light intensity is almost completely absorbed, thereby further weakening the crosstalk effect on the first photodiode 110. Similarly, Figure 4As shown, when the light path from the first pixel unit 13 on the left side of the middle first pixel unit 13 is incident at a large angle to the middle first pixel unit 13, it will pass through the second photodiode 111 in the left first pixel unit 13 more, thereby causing a significant attenuation, and then enter the first photodiode 110 after attenuation through the semiconductor structure layer 11 in the middle first pixel unit 13. After two attenuations, the light intensity is almost completely absorbed, thereby further weakening the crosstalk effect on the first photodiode 110. Therefore, this arrangement can further improve the anti-crosstalk capability of the first photodiode 110.

[0074] As an example, Figure 1 As shown, the pixel unit of the pixel array further includes an optical structure layer 12 to adjust the incident light. Generally, the optical structure layer 12 includes a microlens 122, a color filter 120 and a light-transmitting medium layer 123 from top to bottom, wherein the microlens 122 is bonded to the color filter 120 through an adhesive layer 124. In addition, an optical isolation structure 121 is provided in the light-transmitting medium layer 123 to achieve optical isolation of adjacent pixel units, and the color filter 120 is used to filter selected light of one or more colors.

[0075] As another example, Figure 1 As shown, the semiconductor structure layer 11 of the pixel unit of the pixel array (including the first pixel unit 13 here) can be provided with a shallow trench isolation 113 on the back and a deep trench isolation 112 on the front to prevent charge signal crosstalk between adjacent pixel units, reduce the noise of the image sensor, and improve the accuracy of the image sensor.

[0076] As another example, the first photodiode 110 and the second photodiode 111 may be semiconductor photodiodes formed by an ion implantation process, or may be organic photodiodes, which may be selected according to actual needs.

[0077] like Figures 5 to 8As shown, since the first photodiode 110 is located deeper in the semiconductor structure layer 11 and has a lower sensitivity, its capacity for photoelectrons may be relatively limited. Therefore, a charge overflow capacitor 14 may be used to couple the first photodiode 110 in parallel to increase the capacity. Even if one end of the charge overflow capacitor 14 is grounded together with one end of the first photodiode 110, and the other end is coupled to the other end of the first photodiode 110 and the first transfer gate 100, the photoelectrons accumulated in the first photodiode 110 can overflow into the charge overflow capacitor 14 for storage after the capacity is saturated. Of course, if the capacity of the first photodiode 110 is sufficient, there is no need to additionally set up the charge overflow capacitor 14 to increase the storage space for photoelectrons. In addition, the charge overflow capacitor 14 may be set up in a manner such that it may be a MOS capacitor set in the semiconductor structure layer 11 or a MIM capacitor set in the circuit connection layer 10 (such as Fig.12 The specific options can be flexibly selected according to actual needs.

[0078] The readout circuits for the signals of the first photodiode 110 and the second photodiode 111 may adopt independent readout circuits, that is, the first photodiode 110 corresponds to the first readout circuit arranged in the circuit connection layer 10, and the second photodiode 111 corresponds to the second readout circuit arranged in the circuit connection layer 10, so as to respectively output the data signals corresponding to the first photodiode 110 and the second photodiode 111; or a common readout circuit may be adopted, that is, the first photodiode 110 and the second photodiode 111 share the readout circuit arranged in the circuit connection layer 10, so as to output the data signals corresponding to the first photodiode 110 and the second photodiode 111 based on the readout circuit. The use of independent readout circuits can provide a more flexible timing control method for the exposure and reading of each; the use of a common readout circuit can effectively save space, improve the fill factor, and make the pixel sensitivity higher when designing the pixel unit.

[0079] The following uses a global exposure 4T pixel structure as an example to explain how the first photodiode 110 and the second photodiode 111 use a readout circuit and a common readout circuit, respectively.

[0080] like Figure 5 The figure shows the case where the first photodiode 110 and the second photodiode 111 use readout circuits respectively. The first photodiode 110 corresponds to the first readout circuit 15, and the second photodiode 111 corresponds to the second readout circuit 16;

[0081] The first readout circuit 15 and the second readout circuit 16 each include independent floating diffusions 150 and 160, reset transistors 151 and 161, source follower transistors 152 and 162, and select transistors 153 and 163; wherein,

[0082] The reset transistors 151 and 161 reset the floating diffusions 150 and 160;

[0083] The floating diffusions 150 and 160 store data signals correspondingly transferred from the first photodiode 110 and the second photodiode 111 ; the selection transistors 153 and 163 read out the corresponding data signals through the source follower transistors 152 and 162 .

[0084] The specific circuit connection is as follows: the first electrode of the first photodiode 110 and the second photodiode 111 is grounded, and the other electrode is coupled to the corresponding first transfer gate 100 and the second transfer gate 101 respectively; the first transfer gate 100 and the second transfer gate 101 are coupled to the corresponding floating diffusions 150 and 160 respectively; the first electrode of the reset transistors 151 and 161 is connected to the power supply Vdd, the second electrode is connected to the corresponding floating diffusions 150 and 160, and the gate is connected to the reset signal; the gate of the source follower transistors 152 and 162 is connected to the corresponding floating diffusions 150 and 160, the first electrode is connected to the power supply Vdd, and the second electrode is connected to the first electrode of the corresponding selection transistors 153 and 163; the second electrode of the selection transistors 153 and 163 is connected to the corresponding output signal VLN, and the gate is connected to the selection signal.

[0085] like Figure 7 As shown, a low conversion gain capacitor 164 and a gain control transistor 165 may be provided in the second readout circuit 16, so that the second photodiode 111 has two different charge-to-voltage conversion gains, thereby achieving a larger dynamic range. Specifically, the gain control transistor 165 is coupled between the floating diffusion 160 and the reset transistor 161, one end of the low conversion gain capacitor 164 is coupled to a node between the reset transistor 161 and the gain control transistor 165, and the other end is grounded, and the data signal overflowing from the second photodiode 111 is stored in the low conversion gain capacitor 164 by controlling the opening and closing of the gate of the gain control transistor 165.

[0086] like Figure 6 The first photodiode 110 and the second photodiode 111 share a readout circuit 17, wherein the readout circuit 17 includes a floating diffusion 170, a reset transistor 171, a source follower transistor 172, and a gate transistor 173; wherein,

[0087] The reset transistor 171 resets the floating diffusion 170;

[0088] The floating diffusion 170 stores data signals corresponding to the data signals transferred from the first photodiode 110 and the second photodiode 111 ; the selection transistor 173 reads out the corresponding data signals through the source follower transistor 172 .

[0089] The specific circuit connection is as follows: the first electrode of the first photodiode 110 and the second photodiode 111 is grounded, and the other electrode is respectively coupled to the corresponding first transfer gate 100 and the second transfer gate 101; the first transfer gate 100 and the second transfer gate 101 are coupled to the floating diffusion 170; the first electrode of the reset transistor 171 is connected to the power supply Vdd, the second electrode is connected to the floating diffusion 170, and the gate is connected to the reset signal; the gate of the source follower transistor 172 is connected to the floating diffusion 170, the first electrode is connected to the power supply Vdd, and the second electrode is connected to the first electrode of the selection transistor 173; the second electrode of the selection transistor 173 is connected to the output signal VLN, and the gate is connected to the selection signal.

[0090] like Figure 8 As shown, a low conversion gain capacitor 174 and a gain control transistor 175 may also be provided in the readout circuit 17, so that the second photodiode 111 has two different charge-to-voltage conversion gains, thereby achieving a larger dynamic range. Specifically, the gain control transistor 175 is coupled between the floating diffusion 170 and the reset transistor 171, one end of the low conversion gain capacitor 174 is coupled to a node between the reset transistor 171 and the gain control transistor 175, and the other end is grounded, and the data signal overflowing from the second photodiode 111 is stored in the low conversion gain capacitor 174 by controlling the opening and closing of the gate of the gain control transistor 175.

[0091] This embodiment further provides a back-illuminated image sensor, which includes at least one back-illuminated image sensor pixel structure as described above.

[0092] Embodiment 2

[0093] This embodiment provides a method for preparing a back-illuminated image sensor pixel structure, which can be used to prepare the back-illuminated image sensor pixel structure of Embodiment 1, but is not limited thereto, and other methods that can realize the preparation of the back-illuminated image sensor pixel structure of Embodiment 1 are also acceptable.

[0094] The preparation method of this embodiment is described in detail below with reference to specific drawings, wherein it is taken as an example that all the pixel units of the pixel array are set as the first pixel unit 13 .

[0095] like Fig. 9 As shown, step S1 is first performed to provide a semiconductor structure layer 11 and form a second photodiode 111 in the semiconductor structure layer 11 .

[0096] It should be noted here that the second photodiode 111 formed in the semiconductor structure layer 11 is arranged in an array. The second photodiode 111 can be formed by an ion implantation process, which generally includes: providing a lightly doped semiconductor substrate, which can be a silicon substrate, or germanium, silicon germanium, silicon carbide, gallium arsenide, indium gallium, or a silicon substrate on an insulator surface, a germanium substrate on an insulator surface, or a substrate with an epitaxial layer grown thereon, and other suitable materials for image sensors; then the second photodiode 111 is formed by implanting ions into the semiconductor substrate, and the depth and range of the ion implantation can be controlled by controlling the energy and concentration of the ion implantation. The second photodiode 111 can also be an organic photodiode, which is formed by depositing on a substrate using an existing conventional deposition process, and the material of the organic photodiode uses an existing conventional organic material suitable for image sensors.

[0097] As an example, a shallow trench isolation 113 may be provided between two adjacent second photodiodes 111. The shallow trench isolation 113 is formed by digging a trench and filling it with an insulating material.

[0098] like Fig.10 As shown, step S2 is then performed to form a first photodiode 110 above the second photodiode 111 in the semiconductor structure layer 11 .

[0099] It should be noted here that the first photodiode 110 formed in the semiconductor structure layer 11 is arranged in an array. The first photodiode 110 can be formed by an ion implantation process, which generally includes: providing a lightly doped semiconductor substrate, which can be a silicon substrate, or germanium, silicon germanium, silicon carbide, gallium arsenide, indium gallium, or a silicon substrate on an insulator surface, a germanium substrate on an insulator surface, or a substrate with an epitaxial layer grown thereon, and other suitable materials for image sensors; then the first photodiode 110 is formed by implanting ions into the semiconductor substrate, and the depth and range of the ion implantation can be controlled by controlling the energy and concentration of the ion implantation. The first photodiode 110 can also be an organic photodiode, which is formed by depositing on a substrate using an existing conventional deposition process, and the material of the organic photodiode uses an existing conventional organic material suitable for image sensors.

[0100] like Fig.11As shown, step S3 is then performed to form a circuit connection layer 10 on the semiconductor structure layer 11 , and a first transfer gate 100 and a second transfer gate 101 corresponding to the first photodiode 110 and the second photodiode 111 are formed in the circuit connection layer 10 .

[0101] like Fig.12 As shown, step S4 is then performed to form a charge overflow capacitor 14 in the circuit connection layer 10 and a metal connection structure electrically connected to the semiconductor structure layer 11 .

[0102] As an example, the charge overflow capacitor 14 formed in the circuit connection layer 10 is a MIM capacitor.

[0103] In this step, other circuit structures are also formed on the circuit connection layer 10, such as a source follower transistor, a row selection transistor, a floating diffusion, a reset transistor, etc., which are well-known structures in the art and will not be described in detail here.

[0104] like Fig.13 As shown, step S5 is finally performed to invert the structure and form an optical structure layer 12 on the other side of the semiconductor structure layer 11 .

[0105] As an example, after the above structure is inverted, a deep trench isolation 112 may be formed in the semiconductor structure layer 11 first, and then the optical structure layer 12 may be formed.

[0106] As a specific example, the specific steps of forming the optical structure layer 12 include: first, forming a first light-transmitting medium layer and an optical isolation material layer in sequence on the semiconductor structure layer 11; then, patterning the optical isolation material layer to obtain an optical isolation structure 121, and the optical isolation material layer can be patterned by photolithography and etching processes; then, filling the optical isolation structure 121 with a second light-transmitting medium layer to make the surface of the optical isolation structure 121 flush, and filling the surface of the optical isolation structure 121 to improve the bonding quality of the subsequent bonding color filter; finally, forming a color filter 120 and a microlens 122 in sequence on the optical isolation structure 121 and the second light-transmitting medium layer, and the microlens 122 can be bonded to the color filter 120 by using an adhesive layer 124.

[0107] In summary, the present invention provides a back-illuminated image sensor pixel structure and an image sensor thereof, wherein a first photodiode is disposed below a second photodiode in a first type of pixel unit and the sensitivity of the first photodiode is made smaller than the sensitivity of the second photodiode, so that when a normal light signal passes through an optical structure layer and enters a semiconductor structure layer, most of the light signal is absorbed after passing through the second photodiode and converted into photoelectrons of the second photodiode, while a very small part of the light signal is not absorbed and finally reaches the first photodiode. Therefore, for an image signal with a relatively long stroboscopic period, the exposure time of the first transfer gate can be controlled to ensure that the first photodiode is not overexposed while also preventing the first photodiode from being overexposed. The exposure time is ensured to cover the image signal with a relatively long stroboscopic period, thereby realizing LFM, and for the high-brightness image signal, the exposure time of the second transfer gate can be shortened to ensure that the second photodiode is not overexposed, thereby achieving the synchronous collection of the high-brightness signal and the signal with a long stroboscopic period; in addition, in the pixel array, the light incident at a large angle from the pixel unit adjacent to the first pixel unit to the first pixel unit, due to the deep position of the first photodiode, its considerable optical path is in the photodiode above the first photodiode, and the sensitivity of the first photodiode is relatively low, resulting in the first photodiode hardly absorbing the crosstalk light, thereby effectively improving the anti-crosstalk capability of the first photodiode. Therefore, the present invention effectively overcomes various shortcomings in the prior art and has a high industrial utilization value.

[0108] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A back-illuminated image sensor pixel structure, characterized in that: The pixel structure includes a pixel array in which pixel units are arranged in an array, all the pixel units in the pixel array include a plurality of first pixel units, and the first pixel units are evenly distributed in the pixel array, and the first pixel units include: A semiconductor structure layer, wherein the semiconductor structure layer includes a first photodiode and a second photodiode in sequence from bottom to top, wherein the sensitivity of the first photodiode is smaller than the sensitivity of the second photodiode; A circuit connection layer is formed with a first transfer gate and a second transfer gate, which are respectively coupled to the first photodiode and the second photodiode to transfer data signals accumulated in the first photodiode and the second photodiode in response to transfer signals applied to the first transfer gate and the second transfer gate.

2. The back-illuminated image sensor pixel structure according to claim 1, characterized in that: All the pixel units in the pixel array are set as the first pixel units.

3. The back-illuminated image sensor pixel structure according to claim 1 or 2, characterized in that: A projection of the second photodiode toward the first photodiode completely covers the first photodiode.

4. The back-illuminated image sensor pixel structure according to claim 3, characterized in that: The volume of the first photodiode is smaller than the volume of the second photodiode.

5. The back-illuminated image sensor pixel structure according to claim 4, characterized in that: The surface area of ​​the first photodiode is not greater than 50% of the surface area of ​​the second photodiode, and the depth of the first photodiode is greater than 70% of the depth of the semiconductor structure layer.

6. The back-illuminated image sensor pixel structure according to claim 4, characterized in that: A surface of the second photodiode close to the first photodiode is in a stepped shape, and the first photodiode is located below an inner stepped surface of the stepped second photodiode.

7. The back-illuminated image sensor pixel structure according to claim 1, wherein: The first pixel unit also includes a charge overflow capacitor, and is coupled in parallel with the first photodiode; the charge overflow capacitor is a MOS capacitor arranged in the semiconductor structure layer or a MIM capacitor arranged in the circuit connection layer.

8. The back-illuminated image sensor pixel structure according to claim 1 or 7, characterized in that: The first photodiode corresponds to a first readout circuit arranged in the circuit connection layer, and the second photodiode corresponds to a second readout circuit arranged in the circuit connection layer, so as to respectively output the data signals corresponding to the first photodiode and the second photodiode; or, the first photodiode and the second photodiode share a readout circuit arranged in the circuit connection layer, so as to output the data signals corresponding to the first photodiode and the second photodiode based on the readout circuit.

9. The back-illuminated image sensor pixel structure according to claim 8, characterized in that: When the first photodiode corresponds to the first readout circuit and the second photodiode corresponds to the second readout circuit: The first readout circuit and the second readout circuit each include a floating diffusion, a reset transistor, a source follower transistor and a gate transistor, respectively; wherein, The reset transistor resets the floating diffusion; The floating diffusion stores the data signal transferred from the first photodiode and the second photodiode; the gate transistor reads out the corresponding data signal through the source follower transistor; The second readout circuit further includes a low conversion gain capacitor and a gain control transistor, wherein the gain control transistor is coupled between the floating diffusion and the reset transistor, one end of the low conversion gain capacitor is coupled to a node between the reset transistor and the gain control transistor, and the other end is grounded, so as to store the data signal overflowed from the second photodiode in the low conversion gain capacitor; Or, when the first photodiode and the second photodiode share the readout circuit: The readout circuit includes a floating diffusion, a reset transistor, a low conversion gain capacitor, a gain control transistor, a source follower transistor and a gate transistor; wherein, The reset transistor resets the floating diffusion; The floating diffusion stores the data signal transferred from the first photodiode and the second photodiode; the gate transistor reads out the corresponding data signal through the source follower transistor; The gain control transistor is coupled between the floating diffusion and the reset transistor, one end of the low conversion gain capacitor is coupled to the node between the reset transistor and the gain control transistor, and the other end is grounded to store the data signal overflowed from the second photodiode in the low conversion gain capacitor.

10. A back-illuminated image sensor, characterized in that: The back-illuminated image sensor comprises the back-illuminated image sensor pixel structure according to any one of claims 1 to 9.

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